cmd/internal/obj: use REGENTRYTMP* in a few more places

There are a few remaining places in obj6 where we hard-code
safe-on-entry registers. Fix those to use the consts.

For #40724.

Change-Id: Ie640521aa67d6c99bc057553dc122160049c6edc
Reviewed-on: https://go-review.googlesource.com/c/go/+/307009
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

Author: aclements

src/cmd/internal/obj/x86/obj6.go

@@ -1046,7 +1046,7 @@ func stacksplit(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog, newprog obj.ProgA
 		tmp := int16(REG_AX) // use AX for 32-bit
 		if ctxt.Arch.Family == sys.AMD64 {
 			// for 64-bit, stay away from register ABI parameter registers, even w/o GOEXPERIMENT=regabi
-			tmp = int16(REG_R13)
+			tmp = int16(REGENTRYTMP0)
 		}
 		// large stack: SP-framesize <= stackguard-StackSmall
 		//	LEAQ -xxx(SP), tmp
@@ -1076,8 +1076,8 @@ func stacksplit(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog, newprog obj.ProgA
 		tmp1 := int16(REG_SI)
 		tmp2 := int16(REG_AX)
 		if ctxt.Arch.Family == sys.AMD64 {
-			tmp1 = int16(REG_R13) // register ABI uses REG_SI and REG_AX for parameters.
-			tmp2 = int16(REG_R12)
+			tmp1 = int16(REGENTRYTMP0) // register ABI uses REG_SI and REG_AX for parameters.
+			tmp2 = int16(REGENTRYTMP1)
 		}
 		// Such a large stack we need to protect against wraparound.
 		// If SP is close to zero: